"A lot of people will say that, well, this is a mental-health problem, it's not a gun problem. The United States does not have a monopoly on crazy people. It's not the only country that has psychosis. And yet, we kill each other in these mass shootings at rates that are exponentially higher than anyplace else."

It's foolish to say that in a large complicated society any single factor causes another. But you can look at the composition of that society through key indicators and see how it treats people for clues its behaviour.